Protracted negotiations to conclude a deal have been underway since August following the collapse of an earlier failed bid three years ago, with pensions liabilities forming the main bone of contention stalling progress.

Even though a deal has been agreed it is likely that both companies will be forced to operate as separate entities for a transition period in order to allow the Competition and Markets Authority to assess the impact of one of the most significant consolidations in Britain’s publishing sector for decades.

Lloyd Embley, Trinity Mirror’s editor-in-chief, has already gone on record to warn of job cuts as the publisher makes economies of scale with its hard-won asset, but claims these will be less severe than if the deal had fallen through.

The "brilliant journalists" on the Mirror and Express newspapers must not be casualties of the takeover, the National Union of Journalists warned.

Michelle Stanistreet, NUJ general secretary, said: "After many years of under-investment and one pay increase in the past decade, journalists working for Richard Desmond have been desperate for a new owner to provide the resources needed to help increase the readership and the success of the titles. However, the NUJ is concerned that Trinity Mirror, with its long record of making cuts to its newspapers, will not be the knight on the white horse they were hoping for.

"Therefore we will be seeking guarantees that the deal will not result in redundancies and that the titles will be able to thrive."

For Desmond himself today’s announcement marks the effective end to his 43-year career in publishing after rising to prominence in the 80s after securing the UK rights to publish Penthouse.

OK! Magazine followed in 1993 with Express Newspapers added to his empire in 2000 when Desmond agreed a £125m deal to acquire the tabloids.